There has been known a reactive sputtering device using magnetic fields to form, for example, a titanium nitride film on a substrate to be processed, such as a semiconductor wafer (Hereinafter, referred to as “wafer”).
In this device, by applying a negative DC voltage to a target made of, e.g., titanium and disposed opposite to a wafer, a magnetic field is formed between the target and the wafer by a magnet member provided above the target. If, for example, a processing gas, containing nitrogen gas and argon gas for plasma generation, is supplied to the processing space between the target and the wafer, the target is sputtered by the argon ions while the nitrogen gas is activated, and a titanium nitride film is formed on the wafer.
Specifically, when the concentration of nitrogen gas is relatively low inside the vacuum chamber, the titanium particles (atoms or molecules) are piled down on top of the wafer by the sputtering of the target and then, the titanium particles are nitrided on the wafer by the active species (ions and radicals) of nitrogen gas. Contrarily, when the concentration of nitrogen gas is relatively high inside the vacuum chamber, the surface of the target is nitrided by the active species of nitrogen gas to thereby form a titanium nitride film (layer) which is then sputtered to be deposited on the wafer.
In the case when the target is nitrided (when the concentration of nitrogen gas is relatively high), in comparison to when the concentration of nitrogen gas is relatively low, the electrical resistance becomes large, and thus the negative DC voltage is set to a greater value. Also, at the periphery of the wafer, particles scatter from the target to the periphery of the wafer. Further, since the film thickness gets thinner at the center of the wafer, a high plasma intensity (magnetic field) is set at the center of the wafer in order to compensate for the scattering amount (in order to obtain an in-plane uniform thickness of thin film). In order to obtain the in-plane uniformity of the plasma process, the magnet member, for example, is configured to eccentrically rotate around a vertical axis extending through the center of the wafer.
At this time, the target is provided opposite to the wafer, and thus the processing gas, for example, is supplied from of a side of the processing space in order to suppress the processing gas supply path from interfering with the wafer and the target. Accordingly, the nitrogen gas is more easily consumed at the outer circumference side of the wafer because the plasma intensity is set more strongly at the outer circumference side than at the center of the wafer. In other words, at the processing space, as it becomes easier to generate a non-uniformity in concentration of the active species of nitrogen gas in a radial direction of the wafer, the thin film composition (the ratio of nitrogen contained in the thin film) in the radial direction of the wafer becomes non-uniform, and thus it is difficult to obtain a good yield ratio for chips that are cut out from the wafer.
In the meantime, if the plasma intensity is adjusted to obtain the in-plane uniform composition of thin film, it is difficult to obtain the uniformity of the film thickness. Therefore, it is difficult to simultaneously obtain an in-plane uniform thickness and composition of thin film by using a reactive sputtering device. Also, if the film formation is performed in a narrow gap between the target and the wafer by reducing the gap therebetween in order, for example, to obtain a good film forming rate and a high use efficiency of the target (the ratio of the amount of titanium deposited on the wafer to the amount of the target), it becomes particularly difficult to obtain uniform film thickness and composition.
If the concentration non-uniformity of nitrogen gas forms in the radial direction of the wafer, the center and the periphery of the wafer may diverge away during the titanium nitride film forming process. Specifically, at the center of the wafer, the titanium particles caused by the sputtering of the target and then deposited on the wafer may be nitrided, but, at the periphery of the wafer, particles caused by the sputtering of the nitrided target are deposited on the wafer. In this case, for example, the appropriately applied DC voltage value becomes non-uniform within the surface of the target, so that it becomes difficult to obtain the plasma intensity as initially set.
Therefore, in conventional devices, the distance between the target and the wafer is greatly increased in order to obtain a wider diffusion space of nitrogen gas, or the pressure of the nitrogen gas in the vacuum chamber is set as low as possible in order for the nitrogen gas to spread more quickly. As a result, in the method of using the wider diffusion space, it is difficult to perform film formation within the narrow gap. Also, in the method of setting a low pressure of nitrogen gas in the vacuum chamber, the available pressure conditions for performing film formation becomes limited. In the case where the film formation is performed within the narrow gap, the space in which the nitride gas is scattered is limited and therefore, it is difficult to obtain a good rate of diffusion.
The technique for performing a film formation on a wafer is described in Japanese Patent Applications Publication Nos. 2004-162138, 2000-309867, and 9-118979, but the above-described problem is not dealt with therein.